Various types of automatic damper control devices have been marketed for use with furnaces, hot water heaters and other heating devices for the purpose of minimizing the loss of heat and conserving fuel. Damper control devices are normally designed to close the flue or stack during periods when the furnace is not operating to prevent the stack heat from being dissipated to the atmosphere and to prevent the loss of conditioned air from the room by convection through the stack. When the furnace is operated, the automatic damper control device will move the damper to an open position to permit flow of the waste gases of combustion to the atmosphere.
In one common form of automatic damper control, the damper is moved between the closed and open positions by an electric motor which is operably connected to the room thermostat in the building. When the thermostat calls for heat, the motor is operated to move the damper to the open position. Motor controlled dampers require relatively expensive components, and if the electrical power is shut down the damper control will be inoperative.
Another type of automatic damper control is a pressure responsive type which is used with oil fired furnaces. As shown in the patent to Scott U.S. Pat. No. 1,743,731, the pressure of the draft in the flue will open the damper when the motor blower of the furnace is operated. When the operation of the blower is discontinued, the damper will move by gravity to the closed position.
Automatic damper controls have also been constructed utilizing a temperature responsive mechanism, such as a bi-metallic element, to control the operation of the damper as disclosed in U.S. Pat. No. 3,366,333.